1. Field of the Invention
This invention relates to an image output apparatus, such as a printer or the like, for magnifying an image represented by input image information and outputting the resultant image, and to an image processing apparatus and method for performing resolution transformation from low-resolution information to high-resolution information in communication between apparatuses having different levels of resolution.
2. Description of the Related Art
Various methods for performing resolution transformation from input low-resolution information into high-resolution information have been proposed. In the proposed conventional methods, transformation processing differs depending on the type of image to be processed (i.e., a multivalue image having gradation information for each pixel, a binary image subjected to binary coding using pseudohalftone, a binary image subjected to binary coding using a fixed threshold, a character image, or the like). For example, a closest interpolation method in which, as shown in FIG. 10, the value of a pixel closest to an interpolated point is allocated to the point, and a coprimary interpolation method in which, as shown in FIG. 11, a pixel value E is determined according to the following calculation using the distances from four points surrounding an interpolated point (the pixel values of the four points are represented by A, B, C and D): EQU E=(1-i)(1-j)A+i.multidot.(1-j)B+j.multidot.(1-i)C+ijD
(i and j are the horizontal direction and vertical direction respectively from a point A when the distance between pixels is assumed to be 1 (i.ltoreq.1, j.ltoreq.1)), and are generally used as conventional interpolation methods.
However, the above-described conventional approaches have the following problems.
That is, the method shown in FIG. 10 has the feature that the configuration is simple. However, when a natural image or the like is used as an object image, since pixel values are determined for each block to be magnified, a difference is observed between respective blocks, thereby degrading the quality of the obtained image.
When a character image, a line image, a CG (computer graphics) image or the like is used as an object image, since the same pixel value continues in each block to be enlarged, an inferior image in which a zigzag line called a called jaggy appears particularly at an oblique line is generated. FIGS. 2 and 3 illustrate how jaggy is generated. FIG. 12 illustrates input information, and FIG. 3 illustrates an example of a resolution transformation in which the number of pixels is doubled both in the vertical and horizontal directions according to the method shown in FIG. 10. In general, as magnification increases, the degree of degradation in picture quality increases (each of numerals "200" and "10" in FIGS. 12 and 13 represents the value of a corresponding pixel).
The method shown in FIG. 11 is generally used when magnifying a natural image. Although this method provides averaged and smoothed pixels, edge portions or portions where a sharp image is required become indistinct. Furthermore, for example, in an image obtained by scanning a map or the like, or a natural image containing character portions, important information is missed, in some cases, an the observer due to indistinctness in the generated image caused by interpolation.
FIG. 14 illustrates image information obtained by performing interpolation processing by doubling the input image information shown in FIG. 12 in the vertical and horizontal directions according to the method shown in FIG. 11.
As is apparent from FIG. 14, pixel values are non-uniform and an indistinct image is generated not only for portions in the vicinity of an oblique line but also for the oblique line itself.
Accordingly, the assignee of the present application has proposed, in Laid-Open Japanese Patent Applications (Kokai) Nos. 7-93531 (1995), 7-107268 (1995) and 7-105359 (1995), methods capable of performing resolution transformation without producing an indistinct image due to interpolation and generating a jaggy when forming high-resolution information from low-resolution information.
The basic concept in these proposals resides in removing resolution-dependent components from input original information, increasing the number of pixels in the resultant information so as to correspond to an output resolution, and inferring and forming information corresponding to a new resolution. The resolution-dependent component is removed by performing smoothing processing using an LPF (low-pass filter), and the number of pixels is increased by linear interpolation. When inferring high-resolution information, information after interpolation is subjected to simple binary coding, and pixel values to be output are calculated by performing different types of processing for pixels classified as "1" and for pixels classified as "0".
However, since the techniques disclosed in the above-described patent applications perform smoothing processing using adaptive digital filters, the number of accumulation and summation operations is large, thereby increasing the processing time.
Usually, resolution transformation processing is executed within application software in a host computer, or a printer driver for providing a printer output, and therefore high-speed processing is an important task.
In addition, since the types of adaptive digital filters are limited, it is not easy to perform optimum conversion for all types of image information.